Facebook – Is it still possible to forward messages from Facebook to another account

It’s not exactly a forward per se, but you can use the notifications feature to receive the messages to your email address associated with your Facebook account—not the @facebook.com one, but the @at_your_email_provider.com one—which needs to be set as the primary email address. The problem is that it doesn’t discriminate between emails sent to @facebook.com and Facebook messages, it forwards everything. Also note that it’s not really reliable, sometimes the messages being delivered and sometimes not.

First, it seems that what you experienced is no longer happening.

And, as stated in this answer the limit for a message in Facebook Messenger is 20,000 characters, which is true at the moment.

So, your message exceeds the maximum message length supported, and probably led to an unexpected behavior - the error and situation you described.

Nowadays, as from what I tested when trying to send a message that is too long: If the tab didn't got stuck by pasting the message, and you've been able to sent it, the following notice will appear in red under the sent message:

⚠️ These people aren't receiving messages right now.

Which means that the message wasn't actually sent (because of the exceeding length of it) and so it won't be delivered to the other person. If you'll refresh the page you'll see that the message wasn't sent and doesn't exist.